According to my observations, the muscle groups with long muscle bellies and short tendons grow the fastest when trained frequently and intensely. Those, however, vary for different people.
As I wrote in the article about calves, some lifters will never develop that muscle group to something impressive regardless of training, food intake or drug use because high insertions equal short muscle bellies and there isn’t much room for growth. You should probably still train your calves though at least for strength and ankle health. Ultimately, it’s up to you.
You will notice that your fullest muscle groups will respond quickly to training because they contain more muscle fibers. The higher fiber density results in a higher growth potential. Those muscle groups are also easier to activate during compound exercises.
At the end of the day, it’s all about your personal genetics. For example, my latissimus dorsi muscles have really low insertions. This makes lat training a joy. However, my calves, forearms, hamstrings and arms have ridiculously short muscle bellies, and I know very well that those areas won’t improve much regardless of training routines and experts’ advice.
Since people are different, I can’t tell you which will be the fastest growing muscle on you. In general, the longer the muscle, the better as far as bodybuilding/recreational muscle building is concerned. If you have nice long biceps, they will grow easily compared to other less favorable body parts.
Professional bodybuilders struggle with the same problem and curse their genetics too. Look at Branch Warren’s forearms compared to the rest of him. They are underdeveloped because of the poor insertions. Not that forearms will help him look any better.
Another example would be the bodybuilder Dennis Wolf. He has high lat insertions which create the appearance of a weak and naked lower back. On the other hand, guys like Joel Stubbs, Larry Scott and Dorian Yates have really low lats which make their backs wider and thicker looking.
Favorable insertions are also the reason why Phil Heath looks more “aesthetically pleasing” than his competition. He has full muscle bellies in his entire body and thus no underdeveloped parts. Unlike his competition, he does not have any major weaknesses.
Muscle groups that are more likely to have shorter bellies
Note: Ultimately, it depends on the individual.
Biceps & Triceps
HOW YOU TRAIN IN THE BEGINNING MATTERS MORE THAN YOU THINK
You can only build so much muscle naturally, and that’s why you should choose wisely how to spend your credit. Everybody knows captain upper bodies with underdeveloped legs. Usually, their lower bodies remain on the weaker side forever. This also explains why the “Do you want to be a centaur?” 5×5 squat routines produce so many people with good legs and hips but weak upper bodies.
Of course, the centaur look is linked to high body fat too. Many people who follow the centaur path get fat as hell, and since the legs are a good place to store fat, the extra lard gives the illusion of a bigger lower body even though there isn’t that much muscle.
I believe that the way you train in the beginning always leaves a mark on how you look for the rest of your life.
Be wise, my friend.
UNIVERSAL MUSCLE GROUPS THAT GROW FAST
The trapezius is a muscle that could grow relatively fast for most people. It’s a “brah” made of tough filaments and full of androgenic receptors. In addition, it has very favorable insertions for heavy and frequent training. The blood supply to the area is also very good. As a result, over-training the traps is hard, although not impossible.
In addition, you will rarely see a bodybuilder with weak traps on the professional stage because anabolic steroids make the traps explode right away.
Another body part with similar potential would be the neck. It usually responds fast to training and adapts quickly.
EXERCISE FORM AND SELECTION MATTER TOO
While the basic functions are the same for every human being, your proportions determine which muscle works harder during certain exercises.
For example, the squat is more difficult for people with exceptionally long femurs. Long legs coupled with a short torso make the back squat a hip/glute/lower back exercise. You may be thinking that your legs cannot grow because of genetics when the actual culprit is your technique and/or exercise selection.
There are usually two groups of people when it comes to the upper body – arms or torso dominant.
The torso dominant lifters have a build that makes the chest and back do more work during compound exercises. As a result, those groups develop faster and to larger proportions. The torso guys are mostly taller people with long skinny arms.
On the other hand, people with long and dominant biceps and triceps can use them as bullies during push and pull movements. Those lifters are usually part of the T-rex arms crew.
Muscle groups with short muscle bellies have less potential to grow.
Tall people with longer limbs usually have more body parts with longer tendons, but there are exceptions too.
The traps have a good potential for growth because they allow for frequent heavy training.
The calves, forearms and hamstrings could be very stubborn body parts.